Conventionally, lithographic (litho) inks, and varnishes have been designed to dry via combination of solvent evaporation and oxidative polymerization of unsaturated components.
Consequent to increasing concerns regarding the health, safety, and environmental impacts of volatile organic compounds, (VOCs); the contribution of this component (the evaporation of solvent is presently responsible for the major proportion of conventional litho inks' and varnish's drying speeds) to drying rates is anticipated to shrink in the future, as increasingly stringent limitations are placed on the usage of VOCs. Stacking of fresh prints in order to conserve space, which is typical during long runs, frequently limits the efficacy of solvent evaporation as a drying mechanism. Additionally the incorporation of solvents in litho inks results in undesirable distortion (e.g., capillary spread prior to drying) of applied ink droplets (dot gain), thus limiting print resolution. The extent of dot gain beyond that consequent to the small proportion resulting from the mechanics of imperfect application of ink to a moving substrate tends to be roughly proportional to the percentage of VOCs employed, hence solvent incorporation into litho ink formulations often limits print quality.
The oxidative-polymerization component of the drying of present litho inks almost invariably requires acceleration by toxic heavy metals such as cobalt and manganese, and is often kinetically limited by oxygen availability, especially when printing on essentially non-porous surfaces, and/or when fresh prints are stacked tightly, minimizing air access. A number of technologies have been developed to overcome drying rate limitations of conventional litho inks; these acceleration techniques include, the inclusion of various energy input systems, such as thermal energy (headset, and infrared drying), electron beam (EB), and ultraviolet (UV) radiation. The latter methodology typically requires incorporation of substantial proportions of a combination of expensive, and often-toxic photo initiators, and related auxiliaries, in addition to the expensive hardware, intensive energy consumption, and radiation exposure hazards, implicit in the generation of short wave length radiation used in both EB and UV based curing systems.
U.S. Pat. No. 5,552,467 teaches the usage of thermally activated combinations of chemical reducing agents, and organic (hydro)peroxides, one placed in the ink and the other contained in fountain solution as a means of overcoming the oxidative-polymerization drying component rate limitations of heat set lithographic inks. The patent discloses the use of two-part systems that inherently exclude usage in single fluid inks, and is limited further limited by the reactivity of the components, resulting in relatively short shelf life of solutions and/dispersions of many organic (hydro)peroxides, and of reducing agents in unsaturated oil based vehicles, especially those containing carbon black an/or heavy metal based pigments, and in gum containing fountain solution concentrates. The specified technology has not been claimed to be effective for use in systems which must dry at/or near ambient temperatures.
U.S. Pat. No. 5,173,113 teaches the utility of hydrogen peroxide as a fountain solution additive for the acceleration of drying of lithographic inks, by a factor of approximately twofold. This combination, however, is used as a two-part system, thus inherently precluding use in single fluid inks. This material, often is frequently unstable in the presence of variable valence metals, glycol ethers, nitrogenous materials such as amines and amides, and reducing agents such as aldehydes, ketones, and alcohols; thus liming the shelf life of materials containing same.
It has now been found that he drying rates of most lithographic inks is advantageously enhanced by the utilization of low levels (10 to 20,000 parts-per-million (ppm)) of inorganic peroxy acid salts, (cofactor reducing agents are not required) as replacements for the activated organic (hydro)peroxide-reducing agent combination(s) and/or hydrogen peroxide fountain additive drying accelerators previously disclosed.